Organocatalytic Ring-Opening Polymerization of Cyclic Esters Mediated by Highly Active Bifunctional Iminophosphorane Catalysts

Highly active bifunctional iminophosphorane catalysts have been applied to the organocatalytic ring-opening polymerization (ROP) of L-lactide (LA), delta-valerolactone (VL), and epsilon-caprolactone (CL). LA polymerization using catalyst 2 at 1 mol % loading rapidly gave poly(LA) in full conversion and with excellent control over the molecular weight distribution. VL and CL were polymerized under the control of catalyst 3 at 5 mol % loading. Poly(VL) was obtained in high conversion and with very good control over the molecular weight distribution. The catalyst system was suitable for the formation of short lengths of poly(CL), with good control over the molecular weight distribution. The formation of block copolymers by sequential monomer addition and the use of macroinitiators such as monomethoxy-terminated poly(ethylene glycol) (mPEG) were also demonstrated using the catalyst system. Control experiments using nonbifunctional N-alkyl iminophosphorane 5 demonstrated the roles of both components of the bifunctional catalyst in the ROP reaction. Notably, the bifunctional iminophosphorane catalysts are moisture-stable and nonhygroscopic, enabling the assembly of ROP reactions on the open bench.